For generating ignition in an engine, for example, an internal combustion engine of an automobile, a spark plug has been used to ignite an air-fuel mixture by spark discharge (also simply referred to as “discharge”). Recently, the internal combustion engine is required to satisfy high output and low fuel consumption. A plasma jet ignition plug has been known as an ignition plug having a high ignition property which exhibits fast spreading of combustion and can ignite a lean air-fuel mixture having a higher ignition limit air fuel ratio.
In such a plasma jet ignition plug, a spark discharge gap is formed between a center electrode and a ground electrode when the plasma jet ignition plug is used in a state that the plasma jet ignition plug is configured such that a periphery of the spark discharge gap is surrounded by an insulator made of ceramics, thus forming a discharge space having a small volume that is referred to as a cavity. The plasma jet ignition plug will be explained by taking a plasma jet ignition plug which uses a superimposed-type power source as one example. For example, at the time of igniting the air-fuel mixture, a high voltage is applied between the center electrode, and the ground electrode thus generating spark discharge (also referred to as “trigger discharge”). Due to the dielectric breakdown which is generated in such spark discharge, an electric current of relatively low voltage flows between the center electrode and the ground electrode. Then, by further supplying energy between the center electrode and the ground electrode, a discharge state is changed so as to form plasma in the cavity. Then, by allowing the formed plasma to be jetted through a communication hole (so-called orifice), the ignition of air-fuel mixture is performed. From a viewpoint of jetting of plasma, this stroke corresponds to one stroke.
Japanese unexamined patent application publication No. JP-A-S57-28869 describes a related art plasma jet ignition plug. In the related art plasma jet ignition plug, at the time of forming plasma, it is necessary to set the electric current which flows into the spark discharge gap to an amount that is larger than an amount of electric current which flows for spark discharge in a general spark plug. To increase an amount of electric current which flows into the spark discharge gap, it is necessary to decrease an electric resistance value on a circuit in which the electric current flows, and hence, a resistor is usually not provided on a circuit of an ignition device or in the inside of the plasma jet ignition plug.